Turbocharger whistling is often caused by excessive bearing wear. What generally happens is that the center section bearings tend to wear allowing the impellers to rub against the compressor housing of the Turbocharger causing a whistling during acceleration. This is not to say that all Turbochargers that whistle need to be replaced. Some Turbochargers whistle for years without failure. Frequent oil changes will help prevent Turbocharger failure!